Optical sensors have often been used to measure quantities such as strain, temperature, pressure and displacement. Among the sensors used for this purpose are interferometric-type sensors that rely on beam-splitting techniques to make the desired measurement. These sensors operate on the principle of having one of the split beams (the sensing beam) interact with the environment and then recombining with the other split beam (the reference beam) that has remained isolated from the environment. The sensing beam is modulated as a function of the environment to be measured. The combined beams interfere with each other to produce a pattern that varies with the phase modulation induced by the environment interacting with the sensing beam. Interferometric optical sensors, or interferometers, take a variety of different forms, examples of which are illustrated in FIGS. 1-3.
FIG. 1 shows a Mach-Zehnder type interferometer. In this type of interferometer, a light beam is split and transmitted along two different paths, referred to here as reference leg 3 and sensing leg 4. The sensing leg 4 interacts with the environmental parameter to be measured while the reference leg 3 remains isolated from the environmental parameter. The two beams are then recombined to interfere in an amount dependent upon the phase change induced in the sensing leg 4.
For example, if an optical fiber constituting the sensing leg 4 is bonded to a structure such that strain in the structure causes a corresponding elongation of the optical fiber, the change in the path length will result in a change in the relative phase of the beams traversing the reference leg 3 and sensing leg 4. The relative phase change will yield a change in the output intensity due to the interference of the two beams when they recombine. For a given elongation, the two beams will interfere constructively, resulting in a maximum output intensity. Further elongation of the sensing leg 4 causes the two beams to interfere destructively, resulting in a minimum output intensity. The intensity of the output will thus oscillate between a maximum and minimum value as the length of the sensing leg 4 is varied, forming a fringe pattern. Accordingly, there is a sinusoidal relationship between strain and output intensity.
FIG. 2 shows a Michelson type interferometer. This interferometer works on the same principle as the Mach-Zehnder type interferometer except that in this case the light beams are reflected back from each end of the respective legs 3,4 to be recombined in the same region where the light beam is split, i.e., at the end of the legs 3,4 nearest the light source.
A third known interferometer design is the Fabry-Perot type interferometer shown in FIG. 3. In this design, the sensing and reference legs are co-linear. Two or more reflections are created by in-line partial reflectors. As indicated in FIG. 3, the first reflection 1 occurs at the reflector nearest the light source and the second reflection 2 occurs at the reflector farthest from the light source. The reflectors may be formed by interfaces defined by a gap, or cavity, in the optical path, in which case the interferometer is called an extrinsic Fabry-Perot type interferometer. Here, the first reflection 1 acts as the reference beam and the second reflection 2 acts as the sensing beam. Accordingly, the distances travelled by the reference and sensing beams differ by an amount equal to the round trip distance through the gap. Therefore, a change in the gap length causes fringes to appear at the output.
Extrinsic Fabry-Perot type interferometers are commonly used to measure quantities such as strain, temperature, pressure, and displacement. Such interferometers are advantageous in embedded sensing applications for a number of reasons. For example, extrinsic Fabry-Perot type interferometers offer high sensitivities typical of interferometers while at the same time overcoming many of the common drawbacks associated with interferometers. In particular, extrinsic Fabry-Perot type interferometers are insensitive to polarization and have good thermal stability. The thermal drift associated with these interferometers is expected to be about 0.0002 fringes per 100.degree. C.
Other optical sensors are known which are hybrid sensors based on a combination of two or more optical sensors. For example, a hybrid sensor featuring a Fabry-Perot optical sensor and an intensity-based sensor is described in commonly assigned Ser. No. 07/972,393, the disclosure of which is incorporated herein by reference.
In some technical applications, it is desirable to series multiplex several Fabry-Perot type optical sensors. Unfortunately, optical losses associated with traditional Fabry-Perot type optical sensors severely hamper their ability to be multiplexed in this fashion. Moreover, typical Fabry-Perot type optical sensor designs are also quite fragile. Accordingly, there is a need for a Fabry-Perot optical sensor having a rugged design and for a Fabry-Perot optical sensor which will minimize the losses that occur when such optical sensors are series multiplexed.
Other applications require an optical sensor design exhibiting a wide dynamic range and providing an output signal that is linear with respect to a detected environmental parameter, while at the same exhibiting low sensitivity to temperature and/or vibration. Unfortunately, many fiber-optic-based systems, while having a wide dynamic range, have sinusoidal, rather than linear output signals, and many of these systems are sensitive to temperature and vibration. Thus, there is a need for an optical sensor system having a combination of wide dynamic range, linear output and low temperature and vibration sensitivity.